English abstract: This study was performed on the BIOTA Observatories pair Narais and Duruchaus in a semiarid savanna ecosystem in central Namibia. The two Observatories have been chosen because of their different land-use intensities which is indicated by a strong fence line contrast. The farm of Duruchaus is grazed with high intensity by cattle and sheep, whereas the Narais farm is only lightly grazed. The study had three major goals: i) to identify differences between the Observatories at different spatial scales regarding plant diversity, vegetation composition, and site conditions and to evaluate them with respect to the differing grazing intensities; ii) to give an estimate of the total plant species richness of each Observatory by applying and comparing three different extrapolation methods commonly used in biodiversity research (species-area relationships = SARs; species-sampling relationships = SSRs; non-parametric richness estimators); and iii) to test the influence of time effort on the degree of completeness of floristic inventories at the 1-ha scale. During the rainy season of 2009, all vascular plants were sampled on 20 completely randomly distributed 1000-m² plots on both Observatories. Within each plot, subplots of 0.01 m², 0.1 m², 1 m², 10 m², and 100 m² were nested and replicated. Soil samples were taken and analysed from all 100-m² subplots. Additionally, ten 1-ha plots were sampled in a standardised way with high time effort, registering the point of time, whenever a new species was encountered. A permutation test was used to analyse potential differences in parameter means between the Observatories. SARs were fitted by 12 different models (power, logarithm, asymptotic) and SSRs by six asymptotic models. Further, four non-parametric richness estimators were applied. The curves of species encounters versus sampling time of the 1-ha plots were fitted with the same set of six asymptotic functions as the SSRs in order to determine the 'true' richness within a specific hectare plot. Finally, the species lists for hectare plots were compared to those gained with BIOTA 'standard' sampling in the same year and accumulated over five years. As results, i) there was hardly any difference between the differently grazed Observatories in species richness at the different spatial scales, while other biodiversity indicators and species composition strongly deviated. (ii) The SAR approach with power or quadratic power functions worked best for extrapolation to the Observatory scale (1 km²), while other methods substantially underestimated the total species richness. (iii) The species-time approach proved to be an appropriate tool to specify the total species richness of the distinct area of 1 ha and to provide information on the completeness of vegetation censuses at this scale. Species overlooked during BIOTA 'standard' sampling were mostly rare, low-growing, and/or belong to the group of annual forbs or graminoids.